In alternating current type, motors such as induction motors and synchronous motors, current flowing through each exciting winding is alternating current, whose frequency is proportional to a rotational speed of the motor. Thus, with increasing rotational speed of the motor, frequency of the command current to be supplied to the exciting winding and frequency of the actual current of the exciting winding increase. Frequency increase of the actual current brings amplitude reduction of the actual current with respect to the command, as well as phase delay and power factor reduction.
FIG. 1 is a block diagram showing a conventional speed loop processing used for the speed control of a three-phase AC motor.
In FIG. 1, a reference numeral represents a speed loop compensatory circuit, in which a speed feedback signal of supplied from a speed detector, attached to the motor to detect the actual speed of the motor, is subtracted from a speed command Vc to obtain a speed deviation. The product of the integrated value of the speed deviation and integral constant K1, and the product of the speed deviation and the proportional constant K2 are added, and the result is outputted as torque command Tcmd1. That is, the PI (proportional-and-integral) control is applied to the speed of the motor to obtain the torque command Tcmd1. Next, in order not only to protect the components, such as transistors for controlling the motor but also to protect the motor itself, the torque command Tcmd1 is inputted into a current limiter 2, in which plus-and-minus upper limit values of the torque command Tcmd1 are clamped at predetermined limit values so as to generate a torque command (an amplitude of the current command in the current loop processing) Tcmd2 to be supplied to each phase current loop circuit which controls the current flowing through each phase winding. In a case where the speed loop control is not carried out, the torque command will be directly inputted to the current limiter 2. Then, the torque command (amplitude of the current command) Tcmd2, output by the current limiter 2, is corrected by a rotor position .theta. of the motor detected by the detector and a phase advance amount ph to be determined based on the rotational speed of the motor, and then multiplied by sine values having a 2 .pi./3 phase difference therebetween (3R, 3S, 3T), thereby obtaining torque commands (current commands) Tcmd(R), Tcmd(S) and Tcmd(T) to be supplied to the windings R, S and T, respectively.
Tcmd(R)=Tcmd2.multidot.sin(.theta.+ph) PA1 Tcmd(S)=Tcmd2.multidot.sin{.theta.+(2.pi./3)+ph} PA1 Tcmd(T)=Tcmd2.multidot.sin{.theta.-(2.pi./3)+ph}
In each phase current loop, the IP (integral-and-proportional) control or the PI (proportional-and-integral) control is carried out to control the current flowing through each phase winding.
FIG. 2 is a block diagram showing a conventional control system whose current loop comprises the IP control, corresponding to the R phase of the motor. Control systems for the other phases are substantially the same as that disclosed in FIG. 2. In the drawing, a reference numeral 4 represents an integral element with an integral gain k1, and a reference numeral 5 represents a proportional element with a proportional gain K2. Furthermore, a reference numeral 6 represents a transfer function term of the R-phase winding of the motor, with a winding inductance L and a winding resistance R. A reference character E represents a reverse electromotive force generated by the motor.
A current deviation, obtained by subtracting the actual current Ir from the torque command Tcmd(R), is integrated in the element 4, the integrated value is multiplied by the integral gain K1 and, from this product, the product of the actual current Ir and proportional constant K2 and the reverse electromotive force E are subtracted respectively. The resulting value is supplied to R-phase winding to cause current Ir to flow through the R-phase winding.
As shown in FIG. 2, the current loop comprise, of the 2nd-order control system. Increase of rotational speed of the motor and the resulting increase of the input frequency of each current loop causes a reduction of gain and a generation of phase lag. The phase lag is compensated by the phase lead control using the phase lead correction amount ph as is described above. However, the gain reduction cannot be compensated, so that current amplitude attenuates with increasing rotational speed, thereby causing a decrease of the maximum torque.
As described above, in a current loop, the phase lag and gain reduction occur in the high-frequency region when the motor runs fast. As described previously, since the phase lag can be predicted from the rotational speed of the motor, the phase lag can be compensated by the phase lead control which advances the phase of the command. However, the gain reduction cannot be compensated. For this reason, the amplitude of the actual current becomes small with respect to the command. Therefore, in some cases, the command may be limited even if the amplitude of the actual current is smaller than the limit value of the current limiter.
On the other hand, during deceleration, the motor generates a reverse electromotive force in the same direction as that in which the voltage of the motor is applied, so that sometimes the magnitude of the reverse electromotive force may exceed the limit value of the current limiter. During deceleration, the direction of the current flowing to the motor is opposite to the rotational direction of the motor, but is the same as the direction of the reverse electromotive force. Hence, when the reverse electromotive force is added to the command, the maximum value of the actual current may sometimes exceed the limit value, possibly damaging the control elements, such as transistors, and the motor itself.